Conventionally, for instance, in a hydraulic excavator, a combined operation of a boom lowering operation and an arm pushing operation is performed in order to dump the soil from a bucket.
Normally, a boom cylinder for operating a boom, and an arm cylinder for operating an arm are driven by individual hydraulic pumps (a first hydraulic pump and a second hydraulic pump). On the other hand, when the aforementioned combined operation is performed, part of the oil from the first hydraulic pump for the boom cylinder is caused to merge with the arm cylinder by a merging valve for accelerating an arm pushing operation so as to enhance the work efficiency (see Patent Literature 1).
However, a boom lowering operation is a low load operation, as compared with an arm pushing operation, because the weight of the boom is added during the boom lowering operation. Therefore, in a combined operation of a boom lowering operation and an arm pushing operation, oil from the first hydraulic pump preferentially flows to the low-load boom cylinder.
As a result, redundant oil may flow to the boom cylinder, which may cause power loss. Further, the flow rate of oil to be supplied to the arm cylinder may decrease, which may hinder an intended object of accelerating the operation speed of the arm.
As a countermeasure against the above, there is proposed an idea of increasing the flow rate of oil ejected from the first hydraulic pump. This, however, may be disadvantageous in the point of energy efficiency, because the power of the first hydraulic pump increases.